Polarizing splitter and method for manufacturing same

ABSTRACT

A polarizing splitter includes a base, an asymmetric Y-shaped waveguide, and a pair of strip-shaped electrodes. The Y-shaped waveguide protrudes from the upper surface and includes an input section for the passage of both transverse electric and transverse magnetic waves, a first branch for transmitting the transverse electric wave only, and a second branch for transmitting the transverse magnetic wave only. The first branch and the second branch branch from the input section. The electrodes are positioned on the surface, arranged at opposite sides of the input section, and are substantially parallel with a central axis of the input section.

FIELD

The present disclosure relates to integrated optics and, moreparticularly, to a polarizing splitter having a relatively highpolarization extinction ratio, and a method for manufacturing thepolarizing splitter.

BACKGROUND

Polarizing splitters are used in integrated optics to separatetransverse electric waves from transverse magnetic waves.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the embodiments can be better understood with referenceto the following drawings. The components in the drawings are notnecessarily drawn to scale, the emphasis instead being placed uponclearly illustrating the principles of the present disclosure. Moreover,in the drawings, like reference numerals designate corresponding partsthroughout the several views.

FIG. 1 is an isometric view of a substrate component, according to anembodiment.

FIG. 2 shows the diffusing of a first, a second, and a third materialinto the substrate of FIG. 1.

FIG. 3 shows the forming of a resist pattern layer on a surface of thesubstrate of FIG. 2.

FIG. 4 shows the etching of the substrate of FIG. 3, and removing theresist pattern layer.

FIG. 5 shows the further etching of the substrate of FIG. 4 to obtain amedia grating.

FIG. 6 is a schematic view of the media grating in FIG. 5.

FIG. 7 shows the forming of electrodes on the media grating of FIG. 6 toobtain a polarization splitter.

FIG. 8 shows the forming of a light source on a light incident side ofthe polarization splitter of FIG. 7.

FIG. 9 is a cross-sectional view taken along line IX-IX of FIG. 8.

FIG. 10 is a cross-sectional view taken along line X-X of FIG. 8.

DETAILED DESCRIPTION

The disclosure is illustrated by way of example and not by way oflimitation in the figures of the accompanying drawings in which likereference numbers indicate similar elements. It should be noted thatreferences to “an” or “one” embodiment in this disclosure are notnecessarily to the same embodiment, and such references mean “at leastone.” The references “a plurality of” and “a number of” mean “at leasttwo.”

FIGS. 1 to 7 illustrate a method for manufacturing a polarizing splitter100 according to an embodiment including the following steps.

A substrate 10 is provided. The substrate 10 is made of a birefringentcrystal, such as lithium niobate. The substrate 10 includes a topsurface 102, a bottom surface 106 that is opposite to the top surface102, and a side surface 104. The side surface 104 is perpendicularlyconnected to the top surface 102. In this embodiment, the substrate 10is substantially rectangular and is made of lithium niobate, which canincrease a bandwidth of the polarization splitter 100 as lithium niobatehas a relatively higher response speed.

A first material is diffused onto the top surface 102 and into thesubstrate 10, therefore, a first pre-composed waveguide plate 31 and aninput section 21 are obtained on the substrate 10. In this embodiment,the first material is titanium, and the first pre-composed waveguideplate 31 and the input section 21 can allow transmission of bothtransverse electric waves and transverse magnetic waves.

A second material is diffused onto the top surface 102 and into thesubstrate 10, therefore, a first branch 22 is obtained on the substrate10. In this embodiment, the second material is gallium, and this firstbranch 22 can allow transmission of transverse magnetic waves.

A third material is diffused onto the top surface 102 and into thesubstrate 10, therefore, a second branch 23 is obtained on the substrate10. In this embodiment, the second material is zinc-nickel alloy, andthis second branch 23 can allow transmission transverse electric waves.

The input section 21, the first branch 22, and the second branch 23cooperatively form an asymmetric Y-shaped waveguide 20. The first branch22 and the second branch 23 branch from the input section 21. Aninterface 24 is formed between the input section 210 and the firstbranch 220 and the second branch 230. One end of the input section 21away from the first branch 22 abuts the first pre-composed waveguideplate 31. In this embodiment, the first branch 22 and the input section21 lie in a straight line, and the intersection with second branch 23forms an included angle. A diffusing depth of the asymmetric Y-shapedwaveguide 20 is equal to that of the first pre-composed waveguide plate31.

A resist pattern layer 40 is provided. The resist pattern layer 40 ischromium. The top surface 102 corresponding to the asymmetric Y-shapedwaveguide 20 and a portion of the pre-composed waveguide 31 is coveredby the resist pattern layer 40. The portion of the pre-composedwaveguide 31 covered by the resist pattern layer 40 is rectangular.Exposed portions of the pre-composed waveguide 31 are on opposite sidesof the substrate 10, and the exposed portions of the pre-composedwaveguide 31 are symmetrical in opposition with respect to a centralaxis 212 of the input section 21. The resist pattern layer 40 is formedby a plating, exposing, and developing process.

The substrate 10 together with the resist pattern layer 40 is immersedinto a first etching solution, and the substrate 10 is etched from thetop surface 102 exposed from the resist pattern layer 40 into an innerregion, and then the asymmetric Y-shaped waveguide 20 and the portion ofthe first pre-composed waveguide 31 covered by the resist pattern layer40 protrude from the substrate 10. Thus, the asymmetric Y-shapedwaveguide 20 becomes a ridged asymmetric Y-shaped waveguide. The portionof the first pre-composed waveguide 31 covered by the resist patternlayer 40 forms a second pre-composed waveguide 32. A thickness of thesecond pre-composed waveguide 32 projecting from the substrate 10 isequal to that of the asymmetric Y-shaped waveguide 20 projecting fromthe substrate 10. In this embodiment, the first etching solution ishydrofluoric acid.

Then, the substrate 10 is immersed into a second etching solution toremove the resist pattern layer 40. In this embodiment, the secondetching solution includes nitric acid.

A similar method is used to form a media grating 33, which is formed byetching a portion of the second pre-composed waveguide 32. The mediagrating 33 protrudes from the planar waveguide 34. The remaining portionof the second pre-composed waveguide 32 forms a ridged planar waveguide34. The remaining portion of the substrate 10, except for the planarwaveguide 32, the media grating 33, and the asymmetric Y-shapedwaveguide 20, forms a base 15. The base 15 is configured for supportingthe planar waveguide 32 and the asymmetric Y-shaped waveguide 20. Thebase 15 includes an upper surface 151 parallel with the top surface 102.The asymmetric Y-shaped waveguide 20 is etched so as to be thinner atthe same time the second pre-composed waveguide 32 is etched. In thisembodiment, the thickness of the asymmetric Y-shaped waveguide 20projecting from the base 15 is equal to that of the planar waveguide 32projecting from the base 15.

In this embodiment, the media grating 33 includes an odd number of mediastrips 332 extending along a direction substantially parallel with thecentral axis 212. The media strips 332 are symmetrical about the centralaxis 212. Each of the media strips 332 is rectangular. The thickness ofeach of the media strips 332 is equal. Widthwise, from a center to aside, the total width of each of the media strips 332 decreases, and thewidth of the gap between each two adjacent media strips 332 decreases.

FIG. 6 illustrates that a coordinate system “oxy” is established,wherein the point of origin “o” is an intersection point of the centralaxis 212 and a widthwise direction of the media grating 33, “x” axis isthe widthwise direction of the media grating 33, and “y” axis is a phaseshift of a laser beam 210 at a point “x”. According to wave theory ofplanar waveguides, the phase shift of the laser) beam 21 satisfies theformula: y=a(1−e^(kx) ⁻² ), wherein x>0, a, e, and k are constants. Inthis embodiment, boundaries of the media strips 332 are set to conformto conditions of formulae: y_(n)=a(1−e^(kx) ^(n) ² ) and y_(n)=nπ,wherein x_(n) is the nth boundary of the media strips 332 along the “x”axis, and y_(n) is the corresponding phase shift. That is,

$x_{n} = {\sqrt{\frac{\ln \left( {1 - \frac{n\; \pi}{a}} \right)}{k}}\mspace{20mu} {\left( {x_{n} > 0} \right).}}$

The boundaries of the media strips 332 where x_(n)<0 are determined bythe characteristics of symmetry or otherwise of the media grating 33.

FIG. 7 illustrates a pair of strip-shaped second electrodes 50 parallelwith the media strips 332 and a pair of strip-shaped first electrodes 60parallel with the input section 21. The pairs 50 and 60 are formed onthe base 15 and thereby a polarizing splitter 100 is obtained.

Each strip-shaped second electrode 50 is arranged on opposite sides ofthe planar waveguide 34 and covers a portion of the planar waveguide 34.Each of the strip-shaped second electrodes 50 is slightly longer andslightly higher than each of the media strips 332. Each strip-shapedfirst electrode 60 is arranged on opposite sides of the input section 21and covers a portion of the input section 21. Each of the strip-shapedfirst electrodes 60 is slightly shorter than the input section 21. Thefirst and second electrodes 60, 50 can be formed by, for example,evaporative plating.

FIGS. 8-10 illustrate a laser beam 210 being emitted from a laser lightsource 70. The laser light source 70 is a distributed feedback laser,and is attached to a part of the side surface 104 that corresponds tothe planar waveguide 34 and is aligned with the entrance of the inputsection 21 by, for example, a die bond technology.

The media grating 33 and the planar waveguide 34 cooperativelyconstitute a diffractive waveguide lens to converge the laser beam 210into the input section 21. The strip-shaped second electrodes 50 areconfigured for generating electric fields to change a refractive indexof the planar waveguide 34, thus changing a focal length of thediffractive waveguide lens. Thus, the strength of the laser beam 210that enters into the input section 21 can be adjusted by adjusting thefocal length of the waveguide lens to affect the convergence of thelaser beam 210 at the entrance of the input section 210.

In detail, according to integrated optical theory, the media grating 33and the planar waveguide 34 cooperatively constitute a loadingwaveguide, and the equivalent refractive index of a portion of theplanar waveguide 34 loaded by the media grating 33 is increased. Thus, adifferent type of the diffractive waveguide lens can be obtained bysetting a different structure of the media grating 33. In thisembodiment, the refractive index of the planar wave guide 34 graduallychanges by loading the media grating 33, which is advantageous forobtaining a diffractive waveguide lens with a chirped media grating.

Electric fields {right arrow over (E)}₁ generated by the strip-shapedsecond electrodes 50 traverse the planar waveguide 34 and change anequivalent refractive index of the planar waveguide 34, and thus changea focal length of the waveguide lens. In this embodiment, thestrip-shaped second electrodes 50 cover the planar waveguide 34, thusthe electric fields {right arrow over (E)}₁ are uniformly distributed,and that also increases the overlap area of a transverse electric wavein the planar waveguide 34 and the electric fields {right arrow over(E)}₁ of the strip-shaped second electrodes 50, all of which improvesthe refractive ability of the diffractive waveguide lens.

Due to the birefringency effect, the transverse magnetic wave and thetransverse electric wave traverse the input section 21 separately whenpassing through the interface 24 and respectively enter into the firstbranch 22 and the second branch 23. Electric field generated by thestrip-shaped first electrodes 60 changes a refractive index of the inputsection 21 along a direction substantially perpendicular to the centralaxis 212 of the input section 210. As such, a phase change of thetransverse electric wave is greater than a phase change of thetransverse magnetic wave, which facilitates the separation of thetransverse electric wave from the transverse magnetic wave and increasesa polarization extinction ratio of the polarization splitter 100.

Further, due to the strip-shaped first electrodes 60 covering the inputsection 21, the electric fields {right arrow over (E)}₁ are uniformlydistributed in the input section 21, which also increases a polarizationextinction ratio of the polarizing splitter 100.

It will be understood that the above particular embodiments are shownand described by way of illustration only. The principles and thefeatures of the present disclosure may be employed in various andnumerous embodiments thereof without departing from the scope of thedisclosure. The above-described embodiments illustrate the possiblescope of the disclosure but do not restrict the scope of the disclosure.

What is claimed is:
 1. A polarizing splitter, comprising: a basecomprising a upper surface; an ridged asymmetric Y-shaped waveguideprojecting from the upper surface of the base, comprising an inputsection configured for transmitting both transverse electric wave andtransverse magnetic wave, a first branch configured for transmitting thetransverse magnetic wave only, and a second branch configured fortransmitting the transverse electric wave only, the first branch and thesecond branch branching from the input section; and a pair ofstrip-shaped first electrodes positioned on the upper surface of thebase, arranged at opposite sides of the input section and substantiallyparallel with a central axis of the input section.
 2. The polarizingsplitter of claim 1, wherein the base is made of birefringent crystal.3. The polarizing splitter of claim 1, wherein the first branch and theinput section lie in a straight line while the intersection with secondbranch forms an included angle.
 4. The polarizing splitter of claim 1,wherein the input section, the first branch, and the second branch aremade by diffusing titanium, gallium, and zinc-nickel into a birefringentcrystal, respectively.
 5. The polarizing splitter of claim 4, whereinthe birefringent crystal is lithium niobate.
 6. The polarizing splitterof claim 1, wherein each of the strip-shaped first electrodes isslightly shorter than the input section and aligns with the inputsection.
 7. The polarizing splitter of claim 1, wherein eachstrip-shaped first electrode covers a portion of the input section. 8.The polarizing splitter of claim 1, further comprising: a planarwaveguide formed on the upper surface of the base and connecting an endof the input section opposite to the first branch, the planar waveguidebeing configured to receive a laser beam traversing substantially alongthe central axis and toward the input section; a media grating formed onthe planar waveguide and symmetrical about the central axis; and a pairof strip-shaped second electrodes positioned on the base, at oppositesides of the media grating, and substantially parallel with the centralaxis.
 9. The polarizing splitter of claim 8, wherein the planarwaveguide is made of lithium niobate.
 10. The polarizing splitter ofclaim 8, wherein the planar waveguide is a ridged planar waveguideprojecting from the upper surface of the base, and the thickness of theridged asymmetric Y-shaped waveguide is equal to that of the planarwaveguide.
 11. The polarizing splitter of claim 8, wherein the laserbeam is emitted by a laser light source, which is a distributed feedbacklaser, and is attached to a part of a side surface that corresponds tothe planar waveguide and aligns with the entrance of the input section,and the side surface is perpendicular to the upper surface.
 12. Thepolarizing splitter of claim 8, wherein the media grating is made oflithium niobate diffused with titanium.
 13. The polarizing splitter ofclaim 8, wherein the media grating is a chirped grating.
 14. Thepolarizing splitter of claim 13, wherein the media grating comprises anodd number of media strips extending along a direction that issubstantially parallel with the central axis, each of the media stripsis rectangular, and widthwise, from a center to a side, the total widthof each of the media strips decreases, and widths of the gap betweeneach two adjacent media strips also decreases.
 15. The polarizingsplitter of claim 8, wherein a coordinate axis “ox” is established,wherein the point of origin “o” is an intersecting point of the centralaxis and a widthwise direction of the planar waveguide, and “x” axis isthe widthwise direction of the planar waveguide, boundaries of the mediastrips are set to conform to a condition formulae:${x_{n} = \sqrt{\frac{\ln \left( {1 - \frac{n\; \pi}{a}} \right)}{k}}},$and x_(n)>0, wherein x_(n) is the nth boundary of the media strips alongthe “x” axis, and a, e, and k are constants.
 16. The polarizing splitterof claim 8, wherein each strip-shaped second electrode covers a portionof the planar waveguide.
 17. A method for manufacturing a polarizingsplitter, comprising: providing a substrate, the substrate comprising atop surface; forming a ridged asymmetric Y-shaped waveguide and a baseby etching the substrate from the top surface into an inner region, thebase being thinner than the substrate, the base comprising an uppersurface parallel with the top surface, the ridged asymmetric Y-shapedwaveguide projecting from the upper surface of the base, and comprisingan input section configured for transmitting both a transverse electricwave and a transverse magnetic wave, a first branch configured fortransmitting the transverse magnetic wave only, and a second branchconfigured for transmitting the transverse electric wave only, the firstbranch and the second branch branching from the input section; andforming a pair of strip-shaped first electrodes on the upper surface ofthe base, the strip-shaped first electrodes arranged at opposite sidesof the input section and substantially parallel with a central axis ofthe input section.
 18. The method of claim 17, wherein the step offorming the ridged asymmetric Y-shaped waveguide and the basecomprising: diffusing a first material into the substrate to form theinput section, diffusing a second material into the substrate to formthe first branch, and diffusing a third material into the substrate toform a second branch, and the input section, the first branch, and thesecond branch cooperatively forming an asymmetric Y-shaped waveguide;and etching the substrate to protrude the asymmetric Y-shaped waveguidefrom the substrate, to form a ridge asymmetric Y-shaped waveguide, and aportion of the substrate, except the ridge asymmetric Y-shapedwaveguide, being the base.
 19. The polarizing splitter of claim 18,wherein the first, the second, and the third material are titanium,gallium, and zinc-nickel respectively, and the base is made ofbirefringent crystal.
 20. The method of claim 18, further comprising:forming a first pre-composed waveguide plate by diffusing the firstmaterial into the substrate when forming the input section; etching thefirst pre-composed waveguide plate to from a second pre-composedwaveguide plate when forming the ridge asymmetric Y-shaped waveguide;etching the second pre-composed waveguide plate to form a planarwaveguide and a media grating, a portion of the substrate, except theridge asymmetric Y-shaped waveguide and the planar waveguide, being thebase, the planar waveguide projecting from the upper surface of the baseand connecting an end of the input section opposite to the first branch,the planar waveguide being configured to receive a laser beam traversingsubstantially along the central axis and toward the input section, themedia grating being formed on the planar waveguide and symmetrical aboutthe central axis; and forming a pair of strip-shaped second electrodeson the base, the strip-shaped second electrodes being at opposite sidesof the media grating, and substantially parallel with the central axis.